Recently, there have been high expectations for observation of biological specimens by an atomic force microscope (AFM). The AFM is a device used to observe surface geometry of a microscopic object using a microcantilever with a probe at its tip. Equivalent natural frequency of the cantilever changes due to atomic forces acting between the specimen and probe. Thus, by detecting changes in the natural frequency of the cantilever and calculating the effect of the atomic forces on the probe from the detected changes, it is possible to measure the distance between the probe and specimen. For example, JP2006-208089A discloses a cantilever control apparatus by nonlinear feedback control of cantilever's self-excited vibration in an atomic force microscope
Observation of the real structure of a biological specimen under physiological conditions is one of major goals of submerged AFM observation. However, when an AFM is used in a liquid, since a Q value of the cantilever falls due to viscous damping, a conventional method which uses forced vibration makes observation difficult. To avoid this situation, techniques which use self-oscillation (which is also called self-excited oscillation) have been studied and various techniques have been proposed (refer to the examples, T. R. Albrecht, P. Grutter, and D. Ruger, “Frequency Modulation Detection Using High-Q Cantilevers for Enhanced Force Microscope Sensitivity,” Journal of Applied Physics, Vol. 69 (1991), pp. 668-673; and T. Okajima, H. Sekigushi, H. Arakawa, and A. Ikai, “Self-Oscillation Technique for AFM in Liquids,” Applied Surface Science, Vol. 210 (2003), pp. 68-72).
At low amplitudes, the frequency of the self-oscillation approximately coincides with the natural frequency, making it possible to measure the natural frequency even in liquids. However, during self-oscillation, increases in amplitude with time are typically unavoidable. Thus, in order to use self-oscillation for measuring the biological specimens which will become deformed even on slight impact, it is necessary to keep the amplitude at a sufficiently low level.